Photodiodes comprise of multiple radiation sensitive junctions formed in semiconductor material. Within a photodiode, charge carriers are created by light that illuminates the junction and photo current is generated dependent upon the degree of illumination. Similarly, photodiode array comprises of large number of light sensitive spaced-apart elements, comprising of a semiconductor junction and a region of high response where the photo-generated charge carriers are collected. Array of photodiodes or basically photodiodes are used in various applications including, but not limited to, optical position encoding, and low light-level imaging, such as night photography, nuclear medical imaging, photon medical imaging, multi-slice computer tomography (CT) imaging, radiation detection and ballistic photon detection.
Photodiodes are characterized by certain characteristics, such as electrical, optical, current (I), voltage (V), and noise. Electrical characteristics of photodiode dominantly include shunt resistance, series resistance, junction capacitance, rise or fall time and frequency response. Noise in photodiodes is generated by a plurality of sources including, but not limited to, thermal noise, quantum or photon noise, and flicker noise.
Detection devices are susceptible to numerous radiation damage mechanisms due to increased reverse-bias current and decreased forward voltage over time. Change in doping level, due to radiation damage, adversely affects the width of the depletion region and a decrease in carrier lifetime results in signal loss as carriers recombine while traversing the depletion region.
Also, in certain applications, optical detectors having small lateral dimensions and spaced closely together are favourably produced. For example in certain medical applications, it would be beneficial to increase the optical resolution of a detector array in order to permit for improved image scans, such as computer tomography scans. However, the diffusion length of minority carriers by photon interaction in the semiconductor is in the range of at least many tens of microns in conventional doping levels utilized for diode arrays. Such minority carriers have potential to affect signals at diodes away from the region at which the minority were generated. Therefore, the spatial resolution obtainable may be limited by diffusion of the carriers within the semiconductor itself, even if other components of the optical system are optimized and scattered light is reduced.
Furthermore, another disadvantage of the abovementioned structure of the typical photodiode is for high speed application. Since the cathode contact is located only on the front side, it requires a higher voltage to fully deplete the device and even after the device is fully depleted, under reverse bias the electrons need to travel the undepleted high resistivity zone at the side of the chip to the top contact. The consequence of this is a high series resistance and a low speed due to a high RC-time component of the device. Due to high series resistance and low speed the photodiodes are rendered inappropriate for high speed applications.
In light of the abovementioned disadvantages, there is a need for front side contact, back side illuminated photodiode array having improved characteristics, including high production throughput, low cost manufacturing, uniform as well as high photocurrent density. Further, there is also a need for photodiode/photodiode array having high speed at low biasing voltages.